1. Field of the Invention
The present invention relates to a new synchronous switch. A switch of this type is a static device designed to open or close a load circuit supplied with a rectified alternating or alternating load current in synchronism with a passage of the voltage to a predetermined value close to zero and in response to a control voltage. The intention is thus to synchronize the closing of the switch is such a way that its operation (closing, opening of the circuit) takes place at times when the instantaneous power is relatively low. To this end, the switch is designed to operate in the following manner irrespective of the nature of the load (resistive or reactive):
TO CLOSE APPROXIMATELY AT THE TIME OF THE FIRST PASSAGE TO ZERO OF THE LOAD VOLTAGE FOLLOWING THE APPLICATION OF A CONTROL VOLTAGE,
TO REMAIN IN THE CLOSED STATE UNTIL THE LOAD CURRENT DROPS DOWN EVEN IF THE CONTROL VOLTAGE HAS BEEN REMOVED OF IF THE VOLTAGES AT THE TERMINALS OF THE SWITCH RISE ABOVE THE THRESHOLD VALUE AND,
TO OPEN THE CIRCUIT APPOXIMATELY AT THE TIME OF THE PASSAGE TO ZERO OF THE LOAD CURRENT AND TO RECLOSE AUTOMATICALLY IMMEDIATELY AFTER THIS INSTANT AS LONG AS THE CONTROL VOLTAGE IS MAINTAINED; OR ALTERNATIVELY, TO KEEP THE CIRCUIT IN THE OPEN STATE IF THE CONTROL VOLTAGE HAS BEEN REMOVED UNTIL THE TIME OF THE FIRST PASSAGE TO ZERO OF THE LOAD VOLTAGE FOLLOWING THE POSSIBLE REAPPLICATION OF THE CONTROL VOLTAGE.
2. Description of the Prior Art
The synchronous switches as described in the prior art are generally formed by superposing two electric circuits, an interrupting circuit as such and a control circuit, each of these circuits consisting of one or more semi-conductor elements. U.S. Pat. No. 3,848,158 describes a circuit of this type. In one known type, the switching element is a thyristor, the gate of which is controlled by a control circuit comprising essentially a second thyristor. In this type of circuit, precautions must be taken to avoid the appearance of leakage current in the gates of the thyristors or other semi-conductor elements forming parts thereof. On the other hand, the control circuit supplies a short impulse to the gate of the thyristor which may be insufficient for it to remain in the conducting state for an entire period, for example, when the load placed in the load circuit is highly inductive, which causes a slow initial variation of the current as a function of time.